The first step in determining the potential antiviral activity of compounds is to use an initial or primary screen to identify whether a particular compound has some degree of antiviral activity. Since primary screening often involves large numbers of compounds, it is preferred that the assay system be automated to reduce human resources required and supply costs. Initial screens are typically kept quite simple, using a single method, one virus isolate for each virus group and a single cell line, preferably of human origin. Two basic types of screening assays have been used, depending upon whether the particular virus replicates in tissue culture cells and produces some type of cellular destruction or morphological change (cytopathic effect; CPE). For viruses that produce CPE, such as herpes simplex virus, the most commonly used assay systems are those that measure inhibition of CPE, focus formation, or syncytial formation. Nucleic acid hybridization is also used for viruses such as cytomegalovirus that may take a long time in culture to produce CPE, or for viruses such as Epstein Barr virus that do not produce CPE in cell culture systems but do replicate their DNA. Other assay systems that do not depend on complete viral replication with the production of CPE monitor synthesis of specific gene products such as P24 for HIV and enzymes such as thymidine kinase, DNA polymerase, or reverse transcriptase.
Human herpesvirus 6 (HHV-6) is the causative agent of roseola infantum, a common disease of infancy characterized by high fever and skin rash. This herpesvirus has also been suggested to have a role in mononucleosis, multiple sclerosis, pneumonitis and bone marrow suppression in transplant rejections (Ablashi et al. Human Herpesvirus-6. Perspectives in Medical Virology, A. J. Zuckerman, Editor, 1992, Vol. 4, Elsevier, Amsterdam; Braun et al. Clin. Microbiol. Rev. 1997 10:521-567). HHV-6 is tropic for CD4+ T cells, which are also the natural targets for HIV infectivity. Thus, in AIDS patients, HHV-6 may contribute to the attrition of T cells, which is consistent with its potential catalytic role in HIV infection (Ablashi et al. Human herpesvirus-6. Perspectives in Medical Virology. A. J. Zuckerman, Editor, 1992, Vol.4. Elsevier, Amsterdam.; Braun et al. Clin. Microbiol. Rev. 1997 10:521-567; Lusso, P. and Gallo, R. C. Immunol. Today 1995 16:67-71) and its actual detection in many different necropsied tissues (Corbelliono et al. Lancet 1993 342:1242; Knox, K. K. and Carrigan, D. R. Lancet 1994 343:577-578). HHV-6 has a linear DNA genome of 160 kbp which is now completely sequenced (Gompels et al. Virology 1995 209:29-51); there are 119 open reading frames predicted, of which 67% have counterparts in human cytomegalovirus (HCMV). Although certain structural and functional genes have been identified (Braun et al. Clin. Microbiol. Rev. 1997 10:521-567), little is known about the nature of many HHV-6 gene products and the manner in which the virus is regulated.
Human herpesvirus-8 (HHV-8) is a recently identified herpesvirus which is the apparent causative factor of Kaposil's sarcoma, the most common neoplasm of AIDS patients (Chang et al. Science 1994 266:1865-1869). Accordingly, HHV-8 is oftentimes referred to as Kaposils sarcoma-associated herpesvirus (KSHV). KSHV is also found in Kaposi's sarcoma tumors of non-AIDS patients. The causative role of KSHV in formation of these neoplasms is strongly supported by seroconversion to viral antigens prior to the clinical appearance of Kaposi's sarcoma. KSHV is also associated with body cavity based lymphoma or pleural effusion lymphoma (Cesarman et al. N. Engl. J. Med. 1995 332:1186-1191) and multicentric Castleman's disease (Soulier et al. Blood 1995 86:1276-1280). In Kaposi's sarcoma and pleural effusion lymphoma, malignant cells harbor the virus. Recently, KSHV was suggested to be involved in the bone marrow cancer, multiple myeloma and monoclonal gammopathy of undetermined significance (Rettig et al. Science 1997 276:1851-1854; Said et al. Blood 1997 90:4278-4282; Brooks et al. J. Pathol. 1997 182:262-265; Levy. J. A. Lancet 1997 349:558-563; Neipel et al. J. Virol. 1997 71:4187-4192).
Accordingly, there is a need for rapid screening methods to identify potential antiviral agents specific against these and other human herpesviruses.